1. Field of the Invention
This invention relates to an asynchronous transfer mode (ATM) network switch. More particularly, this invention relates to a switch having means for controlling the flow of ATM cells constituting an individual virtual connection (VC).
2. Background of the Invention
In ATM data transmission, cells of data conventionally comprising fifty-three bytes (forty-eight bytes carrying data and the remaining five bytes defining the cell header, the address and related information) pass through the network on a virtual connection at an agreed upon rate related to the available bandwidth and the level or service paid for. The agreed upon rate will relate not only to the steady average flow of data, but will also limit the peak flow rates.
Over an extensive network, cells on a connection can become bunched together with different cells having different delays imposed upon them at different stages, so that the cell flow on a VC then does not conform with the agreed upon rates. To prevent rates being exceeded to the detriment of other VC's in the network, the network will include, for example at the boundary between different networks, means for policing the flow. The flow policing means typically includes a "leaky bucket" device which assesses the peak and average flow rates of cells on a VC and if required either downgrades the cells' priority or discards cells. An example of such a device is disclosed in co-owned UK Patent Application No. 9505358.3 which is hereby incorporated herein in its entirety. Since policing can result in the discarding of cells which should not be discarded, it is desirable to effect "traffic shaping" to space out the cells on a VC sufficiently so as to ensure that they meet the agreed upon rates, and in particular the peak rates.
A problem with traffic shaping is that it is desirable to delay the transmission of cells by variable amounts in an attempt to avoid cell loss. In practice, however, variable cell delay has been difficult to implement.